Methods of this type, known as image-processing methods of quality control and inspection, are used in various industries. The increase in the number of manufactured goods, the reduction in sizes of some of their components, and in particular of electronic components, and the cost of personnel employed to inspect, are all tending towards the adoption of inspection techniques based on artificial vision and using:
one or more cameras for making images of the goods to be inspected and for delivering electrical signals representative of said images; and
an intelligent electronic system, e.g. microprocessor-based, for processing the electrical signal and for recognizing defects in accordance with a specific algorithm.
There are two main categories of quality control methods based on image analysis:
techniques based on comparison; and
absolute techniques.
Techniques of the first type, based on comparison, are the most frequent for inspecting industrial goods having a high density of geometrical figures, e.g. printed circuits or integrated circuits.
Circuitry is used as the example in describing the prior art and also when describing the present invention. However, the problems and principles involved in performing quality control inspection of circuitry are equivalent to those that may arise in many other fields of endeavour.
The general principle of this method of optical and electronic inspection based on comparison is described in published French application No. 2 321 229 in the name of CIT-ALCATEL (equivalent to U.S. Pat. No. 4,185,298 "Process and apparatus for the automatic inspection of patterns"). The method described in this document relates to inspecting printed circuits. It consists in simultaneously scanning two printed circuit cards in identical manner using two linear cameras. One of the printed circuit cards constitutes a master or reference card and is taken to be free from defects. The other card is inspected by comparison with the master card. Both primary analog signals as generated in parallel by the two cameras are converted into two corresponding secondary logic signals by being digitized in binary form relative to a threshold. The secondary logical signals are then compared electronically, and a defect signal is generated when they do not match.
It may be observed that optical and electronic inspection systems of this type which operate by comparison do not keep any record of the way shapes change. Inspection is performed by comparing a line or a zone on a reference circuit with an equivalent line or zone on the circuit being optically inspected. As a result, the design of the system electronics can be very simple. This explains why such systems were initially rather successful.
However, the main defects of this method lie:
in its very high sensitivity to non-uniformities, to local deformatons, or to overall deformations, any of which may have no significant effect on proper operation of the circuit under inspection; and
great difficulty in distinguishing between detected defects which are serious and other detected defects which are less serious.
That is why improvements have been sought in this first method of optical inspection by comparison, and such improvements are described in several more recent documents, including those mentioned below.
U.S. Pat. No. 4,056,716, IBM, entitled "Defect inspection of objects such as electronic circuits" describes a variant of the comparison method, which does not consist in comparing the image of a circuit with the image of another circuit scanned under the same conditions of accuracy, but which instead consists in comparing the image of the circuit to be inspected with a low resolution image of the reference circuit. This technique serves to reduce the number of false alarms. However, it emphasizes the detection of small defects and is incapable of sorting defects as a function of their environment.
Another attempt at solving the same problem is described in U.S. Pat. No. 4,148,065, HITACHI, entitled "Method and apparatus for automatically inspecting and correcting masks" which describes another variant on the comparison method. In this method, binarization between the reference image and the image under inspection takes place on a minimum number of bits in such a manner that one bit corresponds to the minimum width of the geometrical figures being inspected. The algorithm concerned consists in looking for all shapes smaller than the minimum thickness of a geometrical figure in each of the two images, and in mutually subtracting the defects detected in the two images. The result of the subtraction is considered as being representative of defects. The document also describes a means for limiting errors due to side-effects by applying a zone thresholding technique.
The defect of this method consists firstly in that it cannot detect defects which are larger than the minimum width of a geometrical figure, and in particular large breaks, and secondly that it cannot distinguish between detected defects which are serious and those which are less serious.
Finally, European published patent application No. 0 004 505, THOMSON-CSF, entitled "A system for inspecting a drawing made on a plane support" describes an improvement to the methods of inspection by comparison in that it seeks to avoid the effects due to inevitable offsets between the element under inspection and the reference element. The general process described consists in inspecting each of the circuits line-by-line, in storing a certain number of lines, and in comparing the configuration of pixels on one line not only with the theoretically equivalent configuration of the other circuit, but also with all other configurations thereof which may be slightly offset from the theoretically identical zone.
Despite their conceptual simplicity, methods of optical and electronic inspection by comparison, and in particular those of the type described in the above-mentioned documents, all suffer from the same fundamental defects:
they are highly sensitive to noise (non-uniform lighting, irregularities in the material, structure of the material . . . );
they are incapable of perceiving the seriousness of a defect and of classifying defects by type;
there are difficulties in implementing them and in adapting them to industrial goods of different types, since inspection always requires a reference circuit to be processed in parallel; and
the circuits must be very accurately positioned in order to ensure that their patterns are in register.
That is why a second type of inspection method has been developed which consists in absolute analysis without referring to a reference or master item.
A first variant of this method of absolute inspection is described in U.S. Pat. No. 4,305,097, SIEMENS, entitled "Automated optoelectronic test system for quality control of two-dimensional elements with high geometric figure density". The method consists in analyzing shapes zone-by-zone. I.e., a series of rectangular images are made and these images cover the entire item to be inspected. Each image is digitized, and specific shape processing algorithms are applied thereto, e.g. minimum width rules for conductors and for insulation.
The method described therein is original in that it recommends operation at verious different resolutions depending on the various algorithms which are applied successively. In particular, low resolution images derived from initial high resolution images are used for the application of shape-seeking algorithms, based on comparison with stored reference shapes, thereby detecting special configurations and simple defects. This type of absolute method which operates by analyzing rectangular zones suffers from various defects:
firstly, it must operate on rectangular images, and as a reslt it requires a large amount of memory to be used if good resolution is required;
secondly, it uses algorithms for searching for shapes in a zone, and these algorithms are either not very effective or alse they require excessively long processing times and expensive equipment;
its investigation capacities are low; and
finally, the equipment requires a separate process per type of defect sought.
Further, the shape-seeking algorithms described are difficult to implement electronically since they do not share any repetitive structures. They must be redesigned, as must the processor cards used therewith, each time a new application is to be implemented having shapes of different topology.
A second variant of this method of absolute inspection is described in a document entitled "Automatic copper pattern inspection system for printed wiring boards" published during "Printed circuit convention III" May 22-25, 1984, in Washington. This document describes a printed circuit inspection system having the following characteristics:
by virtue of an optical system having a large depth of field and using a backscattered laser beam, the copper tracks are inspected in three dimensions on two different levels; and
a non-comparative inspection process is provided by coordination between a plurality of sensors disposed in special configurations.
The processing method relies on the fact that if a plurality of sensors are disposed side-by-side in a special configuration to constitute a specially adapted geometrical grid, each type of image defect corresponds to a specific configuration of sensors on the grid. The method then consists in applying specific electronic processing to distinguish various image configurations on the grid of sensors that are characteristic of defects, while the scene is moved relative to the set of sensors.
This absolute method suffers from the following defects:
it requires the simultaneous use of a multitude of sensors, which are not only spaced apart, but which are spaced apart in different directions, thereby giving rise to a system which is very expensive;
further, the image generated by the processed portion must be large in size in order to cover the grid assembly, and such a configuration makes circuit illumination particularly difficult;
also, if a method of specular illumination is used, the object being inspected must be illuminated under a normal incidence, thereby considerably complicating the system;
in addition, the sensor configuration is adapted solely to processing particular shapes, if some other shape is to be processed, the entire sensor configuration must be reconstructed; and
finally, the zone-by-zone processing is slow and complex, such that a very long time (about 7.5 minutes) in required, as specified in the document, for inspecting a 290 mm.times.360 mm circuit with a resolution of 10 microns.
Such slowness of operation is incompatible with the inspection speeds required by modern industry, and in particular by the electronic industry where it is often necessary to inspect thousands of parts daily.
Preferred embodiments of the invention claimed below provide an overall solution for such defects.
Thus, a first aim of the invention is to provide a high-speed method capable of automatically scanning and inspecting the pattern of a two-dimensional scene.
A second aim of the invention is to provide a method of automatically scanning and inspecting a two-dimensional scene with the steps of the method being capable of being implemented very easily using a minimum number of components and a hard-wired electronic processor.
A third aim of the invention is to provide a method of automatically scanning and inspecting a two-dimensional scene without recourse to comparison and capable of inspecting parts having very different patterns without interruption.
A fourth aim of the invention is to provide a method of automatically scanning and inspecting a two-dimensional scene, using a limited amount of memory and a very simple electronic architecture, the method being based on "shape tracking" and enabling the defects encountered to be classified by type.
A fifth aim of the invention is to provide a method of automatically scanning and inspecting a two-dimensional scene and capable of being performed electronically in real time in order to process an image signal at the output from a camera, and in particular at the output from a linear camera operating at a very high acquisition speed.
A sixth aim of the invention is to provide a method of automatically scanning and inspecting a two-dimensional scene, said method being very flexible and adaptable in use, with the method being designed in such a manner that once electronically set up, it is capable of being adapted to a large number of patterns and to a large number of different inspection rules merely by downloading digital tables into the memory of the corresponding processor.
The method of automatically scanning and inspecting the pattern of a two-dimensional scene in accordance with the invention is performed on a line-by-line basis.
Although described below in a highly specific application relating to inspecting printed circuits, the method of the invention is applicable to inspecting any scene having a surface pattern. The sole condition necessary for the method of the invention to be applicable is that the scene must include at least two levels of modulation for a scanning beam of electromagnetic waves. A first level of scene modulation is characteristic of a series of primary shapes which are separate from one another, whereas a second level of modulation relates to a series of secondary shapes occupying the gaps between the various primary shapes.
The method of the invention operates as a function of specific geometrical rules established in advance and stored, in particular, in the form of digital tables, to check both the primary shapes and the secondary shapes of a pattern, to automatically determine those locations in the scene where said rules are infringed, to generate alarms, and to determine the nature of the defects.